This invention relates to flow directing devices for use in gas turbine engines. Specifically, the present invention relates to an apparatus and a method of reducing heat load on an airfoil exposed to a gas flow.
The major components of a gas turbine engine include (beginning at the upstream end, or inlet) a fan section, one or more compressor sections, a burner section, one or more turbine sections, and a nozzle. The engine may also include an afterburner.
Air enters the engine through the inlet, travels past the fan section, becomes compressed by the compressor sections, mixes with fuel, and combusts in the burner section. The gases from the burner section drive the turbine sections, then exit the engine through the nozzle to produce thrust. If present, the afterburner could augment the thrust of the engine by igniting additional fuel downstream of the burner section.
The compressor and turbine sections include a plurality of rotor assemblies and stationary vane assemblies. Rotor blades and stator vanes are examples of structures (i.e., “flow directing structures”) that direct core gas flow within a gas turbine engine. Air entering the compressor and traveling aft through the burner and turbine sections is typically referred to as “core gas.” In and aft of the burner and turbine sections, the core gas further includes cooling air entering the flow path and the products of combustion products.
In and aft of the burner section, the high temperature of the core gas requires cooling of the components that contact the core gas. One such cooling schemes passes cooling air internally through the component and allowing it to exit through passages disposed within an external wall of the component. Another such cooling scheme utilizes a film of cooling air traveling along the outer surface of a component. The film of cooling air insulates the component from the high temperature core gas and increases the uniformity of cooling along the component surface.
Core gas temperature varies significantly within the core gas flow path, particularly in the first few stages of the turbine section aft of the burner section. In the axial direction, core gas temperature decreases in the downstream direction as the distance from the burner section increases. In the radial direction, core gas temperature has a peak at the medial region of the core gas flow path. The radially outer region and the radially inner region of the core gas flow path have the lowest core gas temperatures.
Various flow anomalies can affect the core gas flow. One such flow anomaly is a “horseshoe vortex.” A horseshoe vortex typically forms where an airfoil abuts a surface forming one of the radial boundaries of the gas path, such as the platform of a stator vane. The horseshoe vortex begins along the leading edge area of the airfoil, traveling away from the medial region of the airfoil and towards the stator vane platform. The vortex next rolls away from the airfoil, travelling along the wall against the core gas flow. Subsequently, the vortex curls around to form the namesake flow pattern. The horseshoe vortex detrimentally affects components near the airfoil.
For example, the horseshoe vortex affects the useful life of the wall. Specifically, the horseshoe vortex augments the heat load of the stator vane platform by urging higher temperature medial region core gas flow to the platform. Unlike the airfoil, the platform lacks any cooling schemes that can offset the augmented heat load.
The horseshoe vortex also affects the useful life of the burner section. As discussed above, the horseshoe vortex draws higher temperature medial region core gas flow towards the radial boundary of the gas path. Such heat load augmentation may damage the liner in the burner section since the liner is adjacent (albeit upstream) to the stator vane platform.
Another such flow anomaly is a “passage vortex” that develops in the passage between adjacent airfoils in a stator or rotor section. The passage vortex is an amalgamation of the pressure side portion of the horseshoe vortex, core gas crossflow between adjacent airfoils, and the entrained air from the freesteam core gas flow passing between the airfoils. Collectively, these flow characteristics encourage some percentage of the flow passing between the airfoils to travel along a helical path (i.e., the “passage vortex”) that diverts core gas flow from the center of the core gas path toward one or both radial boundaries of the core gas path. As with a horseshoe vortex, the passage vortex draws higher temperature center core gas flow towards the radial boundaries of the core gas path. This detrimentally affects the useful life of the stator vane platform.
U.S. Pat. No. 6,419,446, also owned by assignee of the present application, is an attempt to prevent horseshoe vortex and passage vortex formation. The patent describes the use of a fillet adjacent the stagnation line of the airfoil. While helping prevent horseshoe and passage vortex formation, the fillet does not reduce the heat load on the airfoil.
A need exists, therefore, for an apparatus and a method of reducing heat load on an airfoil exposed to a gas flow.